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Figure S1. Spearman rank correlation coefficients (abs.) and smoothing lines between altitude, sampling depth (Depth), bulk density of fine earth (BD Fine), bulk soil organic carbon (SOC), bulk soil organic nitrogen (SON), carbon to nitrogen ratio of the bulk soil (Bulk soil CN), bulk soil carbon stock per cm soil (C stock), carbon in particulate organic matter (POC), nitrogen in particulate organic matter (PON), CN ratio of the particulate organic matter fraction (POM CN), particulate organic carbon to bulk soil organic carbon ratio (POC to SOC ratio) and ash content of POM from O horizon soil samples within 82a grassland habitats, Hol, Noway. Units of factors are listed in Table 1. All correlation coefficients (ρ) are significant at P < 0.05. Empty cells in the upper panel indicate no significant correlation. aOne soil plot is not included because of missing data on bulk density data.

Figure S2. Predicted (±SE) (a) total bulk soil organic carbon content (SOC / % of fine earth; n = 17), (b) carbon stock at equivalent soil mass (C-stock eqv. mass / kg m−2; n = 17), (c) carbon stock (volume based) per cm soil (C-stock / kg m−2; n = 17) and (d) particulate organic carbon to soil organic carbon ratio (POC:SOC; n = 17) superimposed on box-whisker plots (medians, 25th and 75th quartile and minimum and maximum values, shown as whiskers) based on the the mean data from each plant community within each sub-enclosure in O-horizon soil samples from grassland habitats within two different plant communities (snowbed and willow-shrub) at three levels of grazing: high = 80 sheep km−2, low = 25 sheep km−2, and no sheep (control), Hol, southern Norway. Predictions are based on fixed effect estimates derived from linear mixed effect models (Table S5) at mean bulk density (g cm−3) (Figure S2a) and mean ash content (Figure S2d) within each treatment combination.

Table S1. Model selection for the fixed effect structure of linear mixed models (based on ML estimation) for percentage carbon of the bulk soil (% SOC), carbon and nitrogen stock based on equivalent mass of soil (kg m−2), carbon stock per cm soil depth (kg m−2), particulate organic carbon to bulk soil organic carbon ratio (POC to SOC ratio) (arcsine-transformed), carbon flux (CO2-C g−1 soil hour−1), carbon flux (CO2-C g−1 POC hour−1) (ln-transformed), bulk density of the fine earth (g cm−3) (ln transformed) and ash content of the POM fraction (%) within two different plant communities (snowbed and willow-shrub; factor ‘pl.comm’) at three levels of grazing (high = 80 sheep km−2, low = 25 sheep km−2, and no sheep; factor ‘treatment’) in grassland habitats (Hol, Norway). Quantitative explanatory variables are BD and ash content of POM. The models always included sub-enclosures (n = 9) nested in blocks (n = 3) as random effects. Steps indicate models with a different fixed effect structure for each dependent variable. AIC = Akaike&apos;s information criterion. ΔAIC = change in AIC betwen the models (negative values indicate improved fit). P is the P-value based on likelihood ratio tests (chi-squared) between two models. P values <−0.05 indicate significantly smaller explanatory power. Step comp. indicates the models compared.

Table S2. Results of linear mixed model analysis (based on REML estimation) for total bulk soil carbon content (SOC / % of fine earth), carbon stock based on equvialent soil massa (C-stock eqv.mass / kg m−2), carbon stock (volume based) per cm soil (C-stock (volume) per cm / kg m−2) and particulate organic carbon to bulk soil organic carbon ratio (POC to SOC ratio) (arcsine transformed) within two different plant communities (snowbed and willow-shrub; denoted ‘pl.comm’) at three levels of grazing (high = 80 sheep km−2, low = 25 sheep km−2, and control (no sheep); denoted ‘treatment’) in grassland habitats (Hol, Norway). Logarithm-transformed bulk density (BD) of the fine earth (g cm−3) and ash content of POM (%) are included as quantitative explanatory variables. Random effects are sub-enclosures (n = 9; factor sub-enclosure) nested in blocks (n = 3; factor ‘block’). The table shows parameter estimates (±SE) for each factor combination and for BD and ash content of POM (column A) and predicted % SOC, C-stock per cm soil and POC to SOC ratio (arcsine transformed) at mean bulk density and ash content of POM (Table 1) for each combination (column B). The model reduction steps (based on ML estimation) are presented in Table S1. Not relevant is marked as ‘−’.

Table S3. Results of linear mixed model analysis (based on REML estimation) for carbon fluxa (µg CO2-C g−1 soil hour−1), carbon fluxa (µg CO2-C g−1 POC hour−1) (ln transformed), bulk density of the fine earth (g cm−3) (BD; ln-transformed) and ash content of POM (%) within two different plant communities (snowbed and willow-shrub; denoted ‘pl.comm’) at three levels of grazing (high = 80 sheep km−2, low = 25 sheep km−2, and control (no sheep); denoted ‘treatment’) within sub-enclosures (n = 9; factor sub-enclosure; random effect) nested in blocks (n = 3; factor ‘block’; random effect) in grassland habitats (Hol, Norway). The table shows parameter estimates (±SE) for each factor combination and for BD and ash content of POM (column A) and predicted carbon fluxes at mean bulk density and ash content of POM (Table 1) for each combination (column B). The model reduction steps (based on ML estimation) are presented in Table S1. Not relevant is marked as ‘−’.

Table S4. Model selection for the fixed effect structure of linear mixed models (based on ML estimation) for percentage carbon of the bulk soil (% SOC), carbon and nitrogen stock based on equvialent soil mass (kg m−2), carbon stock per cm soil depth (kg m−2), particulate organic carbon to bulk soil organic carbon ratio (POC to SOC ratio) (arcsine-transformed), carbon flux (CO2-C g−1 soil hour−1), carbon flux (CO2-C g−1 POC hour−1) (ln-transformed), bulk density of the fine earth (g cm−3) (ln transformed) and ash content of the POM fraction (%) within two different plant communities (snowbed and willow-shrub; factor ‘pl.comm’) at three levels of grazing (high = 80 sheep km−2, how = 25 sheep km−2, and no sheep; factor ‘treatment’) in grassland habitats (Hol, Norway). Quantitative explanatory variables are BD and ash content of POM. The models always included sub-enclosures (n = 9) nested in blocks (n = 3) as random effects. Steps indicate models with a different fixed effect structure for each dependent variable. AIC = Akaike&apos;s information criterion. ΔAIC = change in AIC betwen the models (negative values indicate improved fit). P is the P-value based on likelihood ratio tests (chi-squared) between two models. P-values <−0.05 indicate significantly smaller explanatory power. Step comp. indicates the models compared. Note: Model reductions are based on mean data from each plant community (i.e. snowbed and willow-shrub) within each sub-enclosure.

Table S5. Results of linear mixed model analysis (based on REML estimation) for total bulk soil carbon content (SOC / % of fine earth), carbon stock based on equvialent soil mass (C-stock eqv.mass / kg m−2), carbon stock (volume based) per cm soil (C-stock (volume) per cm / kg m−2), particulate organic carbon to bulk soil organic carbon ratio (POC to SOC ratio) (arcsine transformed), carbon flux (µg CO2-C g−1 soil hour−1), carbon flux (µg CO2-C g−1 POC hour−1) (ln transfomred), bulk density of the fine earth (g cm−3) (BD; ln-transformed) and ash content of POM (%) within two different plant communities (snowbed and willow-shrub; denoted ‘pl.comm’) at three levels of grazing (high = 80 sheep km−2, low = 25 sheep km−2, and control (no sheep); denoted ‘treatment’) in grassland habitats (Hol, Norway). Logarithm-transformed BD of the fine earth and ash content of POM (%) are included as quantitative explanatory variables. Random effects are sub-enclosures (n = 9; factor sub-enclosure) nested in blocks (n = 3; factor ‘block’). The table shows parameter estimates (±SE) for each factor combination and for BD and ash content of POM (column A) and predicted % SOC, POC to SOC ratio (arcsine transformed) and carbon fluxes (µg CO2-C g−1 soil hour−1 and µg CO2-C g−1 POC hour−1; ln transformed) at mean bulk density and ash content of POM for each combination (column B). The model reduction steps (based on ML estimation) are presented in Table S4. Not relevant is marked as ‘−’. Note: Analyses are based on mean data from each plant community (i.e. snowbed and willow-shrub) within each sub-enclosure.

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